Against a backdrop of the proliferation of mobile communication terminals such as mobile phones and increased demand for high-speed data services, it presents a significant challenge to boost the transmission output of base stations and improve the reliability thereof.
To boost the transmission output while at the same time improving the reliability of the base stations, a configuration including two transmission amplifiers is contemplated. One of the reasons is that combining signals amplified by the two amplifiers offers a substantially twice amount of transmission output as compared with a single amplifier. The other reason is that, even in the event of a fault of one amplifier, the other amplifier enables to continue signal transmission although the transmission output is halved, thus ensuring improved reliability.
With such amplifiers, on the other hand, amplification is conducted also in the nonlinear region of the amplification characteristic to obtain the output with high efficiency. Amplification in the nonlinear region of the amplification characteristic, however, results in a nonlinear distortion (distortion component) in the output signal of the amplifier. For this reason, a distortion compensating device is generally provided in the amplifiers to compensate for the nonlinear distortion.
Among such distortion compensating devices is a digital predistorter. The digital predistorter is designed to prepare a characteristic component (distortion compensation component, predictive distortion value) opposite to the distortion characteristic of the amplifier at the input side of the amplifier through digital processing and add this component to the input signal of the amplifier through digital processing. This cancels out the distortion component occurring in the amplifier, thus providing a distortion-free output signal from the amplifier.
Since the amplification characteristic varies from one amplifier to another, the distortion compensation component of the digital predistorter also varies from one amplifier to another. To obtain an appropriate distortion compensation component responsive to the amplification characteristic of the amplifier used, therefore, the amplifier must be operated for more than a given time, thus allowing the predistorter to find the distortion compensation component responsive to the amplification characteristic of the amplifier. For this reason, it takes a given time before the appropriate distortion compensation component is found and the distortion component in the amplifier's output signal is canceled out by the distortion compensation component.
In the configuration described at the beginning in which two amplifiers are provided, a condition occurs equivalent to that in which the amplification characteristic of the amplifiers changes because of a transition from the state where both of the amplifiers operate to the state where, as a result of a fault of one of the amplifiers, only one of the amplifiers operates. In this case, it requires a given time again before the distortion compensation component of the predistorter is changed into a value appropriate for the amplification characteristic of a single amplifier.
During the period of such a transient state, a signal containing a distortion component will continue to be output from the amplifier. This distortion component likely leads to failures including noise in the adjacent channels.